Steel components as represented by gears and bearings are used in severe environments and may be subjected to large loads during torque transmission and the like. Therefore, high surface fatigue strength is required for such steel components.
Steel components are generally produced in the following method. First, a starting material is foamed into a desired shape to produce an intermediate product. The intermediate product is subjected to a case hardening treatment to obtain a steel component. The casehardened steel component has high surface fatigue strength.
A method for increasing surface fatigue strength has been proposed in Japanese Patent Application Publication No. 2013-204645 (Patent Literature 1) in which a surface unevenness is formed on the surface of a steel component by pickling treatment. However, a pickling treatment is added in this method. Thus, the number of processes increases compared with an ordinary production method of a steel component. Increase in the number of processes will lead to increase in production cost.
Another method for improving surface fatigue strength is a method of increasing Si content in a steel component. Si improves hardenability of a steel component and further improves temper softening resistance in martensite. Thus, Si increases strength of a core part of the steel component and also increases surface fatigue strength.
A further method for increasing surface fatigue strength is a method of performing carburizing treatment as the casehardening treatment. Carburizing treatment foams a carburized layer on the surface of a steel component, thereby increasing surface fatigue strength of the steel component.
Japanese Patent Application Publication No. 2008-280610 (Patent Literature 2) discloses a method for producing a steel component having an increased Si content. In Patent Literature 2, a steel containing 0.5 to 3.0% of Si is subjected to a vacuum carburizing treatment. However, performing continuous treatment is difficult in such vacuum carburizing treatment. Moreover, tarring is likely to occur in vacuum carburizing treatment. Further, the properties of a steel component is difficult to control. Therefore, mass production of a steel component is difficult by means of vacuum carburizing treatment, leading to low productivity.
Another carburizing treatment different from the vacuum carburizing treatment is gas carburizing treatment. Gas carburizing treatment does not have the above described disadvantage of vacuum carburizing treatment. Therefore, gas carburizing treatment is suitable for mass production of steel components.
However, Si in steel deteriorates carburizing properties in gas carburizing treatment. For example, a casehardening steel having a chemical composition corresponding to SCr420 specified in JIS G4052 (hereafter, referred to as an ordinary casehardening steel), and a case hardening steel having a higher Si content compared to that of SCr420 (hereafter, referred to as a high-Si steel) are prepared. The ordinary casehardening steel and the high-Si steel are subjected to a gas carburizing treatment under the same condition. In this case, the depth of effective hardened layer of the high-Si steel becomes smaller than that of the ordinary casehardening steel.
It is reported in “IRON AND STEEL,” 58th year (1972), Vol. 7, (Jun. 1, 1972, published by The Iron and Steel Institute of Japan), P. 926 (Non Patent Literature 1) that increase in Si content results in decrease in gas carburized depth. Therefore, there is a need for development of a production method which enables to achieve a sufficient depth of effective hardened layer even when a high-Si steel is subjected to gas carburizing treatment.
A gas carburizing method for increasing fatigue strength of a steel component is disclosed in Japanese Patent Application Publication No. 02-156063 (Patent Literature 3) and International Application Publication No. WO12/077705 (Patent Literature 4).
In Patent Literature 3, a steel material is subjected to preliminary carburization at a carburizing temperature higher than A1 transformation point such that the surface carbon concentration is not less than 1.0%. Next, the steel material is gradually cooled to immediately above the A1 transformation point and is soaked. Next, the steel material is reheated to a temperature less than the carburizing temperature during preliminary carburization and is quenched.
However, steel materials to be addressed in Patent Literature 3 are SCr steel, SCM steel, SNCM steel, and casehardening steels specified in JIS Standard. The Si contents of these steels are low. Therefore, when a steel having a high Si content is subjected to the gas carburizing treatment of Patent Literature 3, sufficient surface fatigue strength may not be achieved.
Patent Literature 4 discloses the following items relating to a production method including gas carburizing treatment of a high-Si steel. When a high-Si steel is subjected to an ordinary gas carburizing treatment, oxide coating is formed on the surface thereof in an early stage of the carburization. The oxide coating deteriorates gas carburizing property. Accordingly, in Patent Literature 4, the following gas carburizing treatment is performed. First, a steel material is subjected to primary carburization under an atmosphere in which oxide coating is generated. Next, the oxide coating formed on the steel material is removed by shot peening and chemical polishing, etc. Next, the steel material whose oxide coating has been removed is subjected to secondary carburization.
However, in the method of Patent Literature 4, a process of removing oxide coating is added compared to an ordinary carburizing treatment. Increase in the number of processes will lead to deterioration in productivity and increase in production cost.